The present invention generally concerns dithiooxamide (rubeanic acid) and substituted dithiooxamide compounds. Dithiooxamides are compounds of the general formula I, as follows: ##STR1## wherein: typically each R is independently H, an alkyl group, or a substituted alkyl group, although other substituents as outlined herein below, on each N atom are possible. When it is said that each R is "independently" one of the named substituents, it is meant that there is no requirement that all groups R be the same.
The "non-N-substituted" member of the family of compounds designated by formula I, i.e., wherein all groups "R" are "H", is generally referred to herein as "dithiooxamide" or by its formula H.sub.2 NC(S)C(S)NH.sub.2. Dithiooxamide is a well-known and widely studied compound. Because it presents four potential sites for coordination (two sulfur atoms and two nitrogen atoms) certain dithiooxamides according to formula I above are good agents for forming coordination compounds with transition metal salts. In particular, certain compounds according to the general formula I have been shown to form coordination complexes with transition metal cations; for example, cations of nickel, zinc, palladium, platinum, copper, iron, and cobalt.
At least because of its relatively high vapor pressure and water solubility, unsubstituted dithiooxamide, i.e., formula I wherein each "R" group is "H", is an inconvenient reagent for use in forming complexes with transition metals under certain circumstances, as for example in pressure sensitive imaging constructions. Also, because of the instability of unsubstituted dithiooxamide in the encapsulation process involved in the manufacture of pressure sensitive imaging constructions, i.e., impact imaging constructions or image transfer constructions, it is an inconvenient reagent for such an application.
In part to avoid such problems, N-substituted analogs have been formed and studied. Some of the better known of these are the N,N'-(disubstituted)dithiooxamide compounds, i.e., compounds according to the general formula II, as follows: ##STR2## wherein: R.sup.1 and R.sup.2 are independently selected from the group generally comprising alkyl groups, substituted alkyl groups, aryl groups, and substituted aryl groups; the above-listed groups including within their scope groups containing heteroatoms, and cycloaliphatic structures. The better known such compounds are symmetrically substituted compounds, i.e., compounds wherein R.sup.1 and R.sup.2 are the same.
Because dithiooxamides possess four possible sites for coordination, they can theoretically form at least three general types of coordination modes with a transition metal "M", as is represented by the following formulae III, IV, and V. ##STR3##
It will be understood that if the dithiooxamide involved in coordination is asymmetric, i.e., is substituted differently at the two nitrogen atoms, then there would be two possible types of coordination represented by formula IV.
Also, when the uncomplexed dithiooxamides have at least two amide protons, with at least one H-atom on each N-atom in the molecules as represented by formulae I or II, at least three general types of coordination compounds or complexes are possible, as is represented by formulae VI, VII, and VIII. ##STR4##
Herein, continued reference will be made to the three different general types of transition metal complexes of dithiooxamides represented by VI, VII, and VIII. The first of these (formula VI) will be generally referred to herein as a "monomer" complex and comprises a coordination complex of two equivalents dithiooxamide per equivalent of transition metal. The second type (formula VII) will be referred to as a "cationic" complex, and the third type (formula VIII) will be referred to as a "polymer".
In the coordination compound generally referred to herein as the "monomer" or "monomer complex" (formula VI), the transition metal generally has an oxidation state of +2, and each equivalent of dithiooxamide has an overall charge of -1, as a result of the removal of one amide proton. This coordination compound will generally be referred to herein as a 2:1 adduct of ligand to metal atom, or according to the formula M(HL).sub.2 typically wherein: M=Ni.sup.+2, Zn.sup.+2, Cu.sup.+2, Co.sup.+2, Fe.sup.+2, Cd.sup.+2, Hg.sup.+2, Pb.sup.+2, Pd.sup.+2, or Pt.sup.+2 ; and, HL refers generally to the "ligand", i.e., unsubstituted or substituted dithiooxamide having a charge of -1, due to removal of a thioamide proton.
Generally, in the representation of the "monomer" (formula VI) the coordinating sulfur atoms are in the same plane. It will be understood that alternative isomers to formula VI are possible. For example, the functional groups could be oriented such that each ligand is coordinated through a nitrogen atom and a sulfur atom to the metal center.
Certain monomer complexes containing N,N'-(disubstituted)dithiooxamides have been described in the literature. For example, in R. A. Dommisse et al., Bull. Soc. Chim. Belg. 1979, 88, 109, bis(N,N'-diisobutyldithiooxamide)Pd(II) and bis(N,N'-diisobutyldithiooxamide)Pt(II) were described; and in H. Hofmans et al., Bull. Soc. Chim. Belg. 1985, 94, 705, the authors reported the crystal structure of bis(N,N'-dicyclopropyldithiooxamide)Pd(II). In general, formation of a monomer complex requires maintaining a ratio between the ligand and the cation of at least two to one in neutral media.
The second type of coordinating complex involving dithiooxamide is referred to herein as a "cationic" complex. The coordination is generally represented by formula VII above and comprises a complex wherein: the transition metal cation has an oxidation state of +2; the dithiooxamide ligand is neutral; there are two dithiooxamide ligands per equivalent of metal cation; and, there are two anions (for example, halogen anions) associated with the complex. It will be understood that formula VII is merely a representation of the stoichiometry of the cationic complex, and does not necessarily require that the ligands labeled "X" be coordinated trans to each other or that they be directly coordinated to the metal. Also, while coordination of the ligand is shown as involving sulfur atoms only, alternative coordination may be possible.
Herein, the cationic complex is referred to by the general formula M(H.sub.2 L).sub.2 X.sub.2 wherein: M is a transition metal cation having a valence of +2, for example, Ni.sup.+2, Zn.sup.+2, Pd.sup.+2, Pt.sup.+2, Cu.sup.+2, Fe.sup.+2, Cd.sup.+2, Hg.sup.+2, Pb.sup.+2, or Co.sup.+2 ; H.sub.2 L is a dithiooxamide ligand having two amide hydrogens therein, with one H-atom on each amide nitrogen; and, X is an anion having a charge of -1, for example, Br.sup.-, Cl.sup.-, F.sup.-, or I.sup.-. Cationic complexes have been described in, for example, M. R. Green et al., Inorg. Chem. 1987, 26, 2326 and G. C. Pellacani et al., Inorg. Chim. Acta 1974, 9, 189.
The third complex of interest herein is referred to as a "polymer" complex, and generally comprises a complex with about a 1:1 ratio of ligand to transition metal cation. The actual ratio of ligand to metal is typically, however, a little greater than 1:1, since the "polymer" complex may comprise low molecular weight oligomers, and therefore generally includes more ligands, since they are end groups, than metal cations. The ligand has an overall charge of -2 due to removal of two amide protons, i.e., one from each amide group, and the transition metal cation has an oxidation state of +2. One theoretical structural formula for such a polymer complex is generally represented above by formula VIII.
It will be understood that alternative coordination is possible and that the structure shown in formula VIII merely represents the general stoichiometry, and one possible structure, of the polymer. That is, the polymer need not necessarily possess the symmetry shown in formula VIII. For example, the polymer would still be possible in isomeric structures wherein each dithiooxamide ligand does not necessarily have one nitrogen and one sulfur coordinated with each associated metal cation.
The polymer complex will be generally referred to herein as such, or according to the general formula (ML).sub.n wherein: M is a transition metal cation having an oxidation state of +2, for example Ni.sup.+2, Zn.sup.+2, Pd.sup.+2, Pt.sup.+2, Cu.sup.+2, Fe.sup.+2, Cd.sup.+2, Hg.sup.+2, Pb.sup.+2, or Co.sup.+2 ; and, L is a dithiooxamide ligand having a charge of -2, i.e., a dithiooxamide or substituted dithiooxamide molecule having one amide hydrogen removed from each of the amide groups. The designation "n" is merely an integer, indicating polymer structure.
Some polymer complexes involving substituted dithiooxamides are known. In general, the known complexes often involve symmetrically disubstituted dithiooxamides according to the general formula II above. Such polymer complexes are frequently characterized by their exhibition of a deep color. In particular the Ni.sup.+2 polymer complex of symmetrically disubstituted dithiooxamides (where the groups R.sup.1 and R.sup.2 in formula II are aliphatic) are often magenta, purple, or red. Examples of polymer complexes containing substituted dithiooxamide ligands are disclosed in R. N. Hurd et al., J. Am. Chem. Soc. 1960, 82, 4454; H. Hofmans et al., Spectrochim. Acta 1982, 38A, 1213; and R. N. Hurd, Review of the Scientific and Patent Literature on Dithiooxamide, its N-Substituted Derivatives and Their Metal Complexes, Mallinckrodt Chemical Works, St. Louis, Mo., 1963.
The chemistry and characteristics of dithiooxamide compounds have been exploited commercially. For example, dithiooxamide compounds, with transition metal cations having a +2 valence state, have been used in the preparation of carbonless image transfer products or constructions. Carbonless imaging constructions, or products employing this chemistry, generally involve placement of one reactant (i.e., one of the transition metal or ligand material) on one substrate (for example, sheet of paper) and the other reactant (the one of transition metal or ligand material not used on the first substrate) on a second, i.e., mating, substrate. The ligand material and metal are maintained separated from contact and reaction with one another. This is typically accomplished by encapsulation of one of the reactants. Herein, the terms "encapsulation" and "encapsulated compounds" refer to microcapsules enclosing a fill material therewithin.
Once rupturing pressure is applied to the construction, as from a stylus or business-machine key, the encapsulated reactant is released, and a complex between the previously separated reactants is formed. In general, the resulting complex will, of course, form a colored image corresponding to the path traveled by the stylus, or the pattern of pressure provided by the key.
In one commercial product, the capsules on a first sheet (donor sheet) contain dithiooxamide (DTO) derivatives, and the mating sheet, sometimes referred to as the receptor sheet, contains a coating of selected salts of nickel. The encapsulated dithiooxamide compounds, in a suitable binder, are coated onto one face of the donor sheet; and, the metal salt, optionally in a suitable binder, is coated onto one face of the receptor sheet. Herein, the term "suitable binder" refers to a material, such as starch or latex, that allows for dispersion of the reactants in a coating on a substrate, and is readily rupturable under hand-held stylus pressure, or typical business machine key pressure. When the two coated faces are contacted such that the dithiooxamide compound and the metal salt can combine and react, a coordination complex forms and an image results. Typically, this occurs by transfer of the ligand material to the site of the metal salt, i.e., transfer of the ligand material from the donor sheet to the receptor sheet. The image, of course, forms on the receptor sheet.
In a preferred orientation, the encapsulated ligand material, in a suitable binder, is coated on the back of the donor sheet, sometimes referred to as a coated back (CB) sheet, and the metal salt, optionally in a suitable binder, is coated on the front of the receptor sheet, or coated front (CF) sheet. Again, in imaging, the two sheets are positioned such that the encapsulated ligand material on the donor (CB) sheet faces the metal salt coating on the receptor (CF) sheet. When pressure is applied to the uncoated surface of the donor sheet, i.e., the face nor in contact with the receptor (CF) sheet, selected capsules rupture (i.e., those capsules corresponding to the pattern of applied pressure) with release of the ligand material for transfer to the receptor sheet, forming a colored pattern due to complexing with the salt. In many applications the uncoated surface of the donor (CB) sheet comprises a form of some type. The stylus pressure is generated by means of a pen or other writing instrument used in filling out the form. Thus, the image appearing on the receptor (CF) sheet is a copy of the image applied to the top sheet.
In some applications, multiple form sets have been used. These contain intermediate sheets having a metal salt coating on one side and a coating with capsules of ligand material on the opposite side. Such sheets are generally referred to herein as "CFB" sheets (i.e., coated front and back sheets).
Due to the stoichiometry of the system (i.e., the metal salt is usually in excess since relatively little of the encapsulated ligand material is released), it is generally believed that the image formed on the receptor sheet, after stylus pressure is applied to break the capsules and release the encapsulated compound, results from the polymer complex. That is, the color generally results from (ML).sub.n material, but is not necessarily limited to that. The monomer complex, M(HL).sub.2, may also be present in small amounts, or as an intermediate in the formation of the polymer. The anion of the transition metal salt, which is usually the conjugate base of a weak acid, may facilitate removal of the two amide protons from the substituted dithiooxamide compounds, necessary for polymer complexation with the M.sup.+2 cation.
Certain dithiooxamide materials have been utilized in commercially available carbonless paper products. Generally, they comprise symmetrically disubstituted dithiooxamide compounds and include N,N'-dibenzyldithiooxamide and N,N'-di(2-octanoyloxyethyl)dithiooxamide.
In commercial applications, generally, nickel salts have been preferred as the transition metal salts. One reason for this is that nickel salts form a deep color when complexed with the dithiooxamide ligands. The nickel salts are also substantially colorless, and thus do not alone impart color to the receptor (CF) sheet. A third reason is that nickel salts are relatively low in cost, by comparison to other transition metal salts that can be easily and safely handled and that form highly colored coordination complexes with dithiooxamides.
It is desirable that the color of the complex be a deep, strong color that is not only pleasing to the eye, but that will exhibit good contrast with the paper, for purposes of later reading and/or photocopying. This has been one drawback with conventional carbonless paper arrangements, which use nickel salts complexed with disubstituted dithiooxamide ligands. The image imparted by the resulting coordination compound, under such circumstances, is generally magenta. The more "red" character the polymer complex exhibits, generally, the less contrasting and pleasing is the appearance. A dark, i.e., preferably black, blue, or blue-black, arrangement would be preferred, but previously such has not been satisfactorily obtainable.
In conventional impact imaging constructions, the capsules can be inadvertently ruptured in steps such as processing, printing, cutting, packaging, handling, storing, and copying. In these situations inadvertant marking or discoloration (i.e., backgrounding) of the sheets results due to inadvertant capsule rupture and transfer of the encapsulated material to the mating sheet where color formation occurs. The amount of inadvertant backgrounding has been reduced in such products by the use of a color control coreactant distributed externally among the capsules. This coreactant is capable of reacting with the contents of the ruptured capsules before transfer of said contents to the receptor sheet and formation of an undesired image. See D. A. Ostlie, U.S. Pat. No. 3,481,759 (1969).
The dithiooxamide compounds generally useful in carbonless paper constructions should be relatively nonvolatile, so that free dithiooxamide compounds, which would result from any inadvertently ruptured capsule, does not readily transfer from the donor sheet to the receptor sheet and form undesired spots of imaged area. That is, so that without the specific assistance of stylus or key pressure, transfer is not readily obtained. Also, preferably the encapsulated ligand material should be colorless, since the ligand material is often encapsulated and coated on the backside of a sheet, such as a form, which has printing on one or both sides thereof. This aspect is particularly important if the donor sheet comprises a top sheet for a stack of carbonless papers. Such sheets are often white, so that they can be readily identified as originals, can be readily photocopied, and can be easily read.
It is also desirable that the ligand material be capable of being encapsulated and of rapidly forming a stable colored image upon contact with the metal cation on the receptor sheet. That is, the transition metal complex should form nearly instantaneously, so that the image is rapidly formed as the stylus pressure is applied to the backside of the donor sheet. This will help ensure formation of an accurate, almost instantly readable, copy. The image should also be relatively stable so that it does not substantially fade with time.
While the above-described preferred characteristics have long been desirable, they have not been satisfactorily achieved with conventional reactants and conventional constructions. What has been needed has been suitable materials and arrangements for achieving the desired features described.